Trait - Child of the Temple
 

I saw this same problem posted a few years back, but with no resolution. On both PC & iPad versions of HL, after selecting the Child of the Temple trait, I should be able to select either Knowledge: Religion or Knowledge: Nobility/Royalty. Problem is, the PC version says 'Nothing to Select', and the iPad version simply has nothing in the drop down menu. Now I have an error on my character portfolio until I select something.

Is at least one of those knowledge skills added to the character?

The dropdown box only displays skills if it exists on your character. Many things in HL works this way.

First add the Knowledge skill to your character. Then from the trait select the skill. :)